In recent years, road-to-vehicle communication systems and vehicle-to-vehicle communication systems using 25 GHz band, 60 GHz band, and so forth, have been studied. In these communication systems, since the antenna apparatus installed in a vehicle is positioned comparatively close to the road (inside a bumper, for example), there is a problem that reflection from the road cannot be ignored and transmission quality degrades due to fading. Therefore, the antenna apparatus installed in a vehicle needs to have narrow directivity to the vertical direction with respect to the road, and to have a small configuration. It is also preferable that the antenna apparatus has high gain in order to extend the communication distance, and can switch the beam direction within the horizontal plane according to the course of the road since a road is assumed to curve as well as to run in a straight line.
Up to now, a planar patch array antenna has been known as an antenna for achieving high gain with a small, planar configuration. This antenna narrows beam directivity in the main radiation direction and achieves high gain by having a plurality of antenna elements arranged within a plane perpendicular to the main radiation direction and performing distribution feeding (see Patent Document 1, for example.)
Also, a patch Yagi-Uda array antenna has been proposed as an antenna capable of switching beam (see Patent Document 2, for example). FIG. 20 is a drawing showing the configuration of a patch Yagi-Uda array antenna described in Patent Document 2. This antenna is configured with feed elements 2001a through 2001d, passive element 2002, and passive element groups 2003a through 2003d, and achieves small antenna apparatus size by sharing the wave directors (passive elements) that account for the major part of a Yagi-Uda array antenna. By switching power feeding to feed elements 2001a through 2001d, it is possible to switch beam in four directions with the antenna apparatus shown in this figure.
A detour element loaded loop antenna has been proposed as another beam switching antenna (see Non-Patent Document 1, for example.) FIG. 21 is a drawing showing the configuration of the detour element loaded loop antenna described in Non-Patent Document 1. As shown in the figure, linear elements 2101a through 2101d are arranged in a rhombic shape, linear detour element 2102a is connected between linear elements 2101a and 2101c, and linear detour element 2102b is connected between linear elements 2101b and 2101d. Feed point 2103a is provided between linear elements 2101a and 2101b, and feed point 2103b is provided between linear elements 2101c and 2101d. Plate reflector 2104 is arranged parallel to the antenna elements configured as described above. By configuring an antenna apparatus in this way, and switching between feed points 2103a and 2103b, it is possible to switch the main beam in two directions. By this means, it is possible to switch beam with a planar, small configuration.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI6-334434    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-142919    Non-Patent Document 1: Technical report A-P2003-157 of The Institute of Electronics, Information and Communication Engineers, November 2003